The present invention relates to cast products having internal features and more particularly, although not exclusively, a casting process for producing products having cooling passages therein.
There are a number of machine components for which it is necessary to provide internal features such as cavities or passages. The complexity of such internal features provides a technical challenge when the intended component is manufactured by casting.
The provision of cooling passages for components which operate in use within high temperature environments is one example in which such complex internal passages are required. Cooling of components is of particular importance for high temperature gas turbine engines in order to ensure that components within the engine are maintained at a suitable operational temperature without deterioration to performance. It is widely acknowledged that the use of internal cooling channels can allow components to operate effectively in hot environments which exceed the melting temperature of the component material.
It is known to provide cooling arrangements in which coolant flow cascades between a plurality of cooling chambers in order to maximise the cooling efficiency and effect. The cascading of cooling flow is used to ensure successive impingement of the coolant flow onto surfaces to be cooled. This technique may be suitable for a number of different types of components and is applied to rotor rims for turbines in a gas turbine engine. Cooling in this manner typically requires a plurality of successive cooling chambers to be defined by internal wall formations in the component. Flow between those chambers is permitted by the provision of openings in the walls such that flow entering a first chamber passes into a second chamber via said openings and then into a further chamber from the second chamber by virtue of further openings. The openings are arranged such that the flow impinges on the surfaces to be cooled in the relevant chambers prior to passing into another chamber.
Whilst such cooling passages are preferable from an operational point of view, the formation of such chambers and openings by way of casting or moulding is a complex process. In an investment or ‘lost wax’ casting process, a core is required which defines the shape of the interior of the component. The core is removed to leave the negative internal space within the component once formed. However a problem exists in that exit apertures must be provided in the component in order to allow removal of the core.
Additional problems arise due to the intricate nature of the core used to define the internal features of the component. The shape of a core which is suited to providing cooling chambers separated by relatively thin walls typically results in a delicate structure which may not be capable of supporting its own weight. A support in the form of a spine is often required to hold the core bodies in a fixed relative position and to maintain tolerances relative to the cast.
The spine is a manufacturing feature and, once removed, leaves unwanted apertures in the final component.
Exit apertures due to removal of a spine and/or the core itself are undesirable in the final component and can cause short circuits or otherwise prevent correct operation of the internal cooling network. Accordingly these passages need to be closed in the final component. Conventional methods of closing the exit apertures involve brazing or welding of closures, which methods are time consuming and can cause detrimental thermal stresses in the final component. Repeated thermal loading of the component can lead to problems on account of thermal stresses, such as cracking or component failure.